The invention relates to a method for commanding a system controlled by means of a time-division multi-level command. The invention consists in acquiring two measurements by means of the sensor, each during a period, the two periods being dissymmetrical relative to the division of the command, determining an offset of the control subsystem and a corrected response without offset of the system to the command as a function of the measurements and of the measurement periods. With the aid of these two measurements, the invention makes it possible to eliminate the effect of the offset in the control subsystem of the system.
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1. A method for commanding a system controlled by means of a time-division multi-level command, the method using a device receiving a setpoint and comprising a control subsystem in which a sensor measures a characteristic variable of the system which changes as a function of the setpoint, the measurement of the variable being capable of modifying the command of the system through the control subsystem, the method comprising the following operations:
acquiring two measurements by means of the sensor, each measurement having a duration extending over a period, the two periods being dissymmetrical relative to the division of the command,
determining an offset of the control subsystem and a corrected response without offset of the system to the command as a function of the measurements and of the measurement periods.
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This application is a National Stage of International patent application PCT/EP2008/063119, filed on Oct. 1, 2008, which claims priority to foreign French patent application No. FR 07 06900, filed on Oct. 2, 2007, the disclosures of which are hereby incorporated by reference in their entirety.
The invention relates to a method for commanding a controlled system. This method is also called servo control or closed-loop control. In a servo control, a command device comprises a control subsystem comprising a sensor measuring a characteristic variable of the system evolving as a function of a setpoint and a comparator measuring a difference between the setpoint and the value of the variable. The sensor is also called a control sensor. The system is commanded by the difference measured by the comparator.
In a servo control, the accuracy of the response of the system to an evolution of the setpoint is usually good. Specifically, the control tends to cancel out the difference between the setpoint and the value of the characteristic variable of the system monitored by the control sensor.
Nevertheless, the control subsystem may sustain an offset, tending to define a non-zero error even if the response of the system is perfectly suited to the setpoint. The offset may be due to the accuracy of the components of the control subsystem. The offset may evolve over time as a function of distinct parameters of the setpoint influencing the response of the system, parameters such as for example the evolution of the ambient temperature or else the wear of the components of the control subsystem.
The problem is currently solved by acting on each of the offsets of the various components and by trying to minimize their value by optimizing the design. Although costly because of the optimization necessary, this solution may be satisfactory at a given time, but does not prevent the evolution of the offset over time.
The invention can be applied in a backlighting command of liquid crystal screens used on aircraft instrument panels where it is necessary for the pilot of the aircraft to be able to see these screens irrespective of the ambient light in the cockpit.
In addition to the electronic offsets specific to any measurement subsystem, the preponderant offset for this backlighting application is generated by the ambient lighting in the cockpit, notably when the sun lights up the liquid crystal screen. A sufficient fraction of ambient light is then measured by the internal lighting sensor and skews the measurement of the latter. Since this interference lighting is added to that generated from the backlighting light source, the accuracy of the brightness seen by the pilot is degraded.
An object of the invention is to improve the robustness of the command of a controlled system by no longer seeking to minimize the offset but by measuring it in order to be able to compensate for it.
The invention is suitable for a system controlled by means of a time-division multi-level command. Specifically, it will take advantage of the various command levels of the system to allow the offset to be measured.
Accordingly, the subject of the invention is a method for commanding a system controlled by means of a time-division multi-level command, the method using a device receiving a setpoint and comprising a control subsystem in which a sensor measures a characteristic variable of the system which changes as a function of the setpoint, the measurement of the variable being capable of modifying the command of the system through the control subsystem, the method being characterized in that it comprises the following operations:
The measured offset can be calculated and subtracted from the measurements acquired by the control sensor by a system of two equations with two unknowns, the two unknowns being the response of the system and the offset of the measurement subsystem, the two equations being the measurements expressed as a function of the two unknowns. The two equations are not redundant if the two periods are dissymmetrical relative to the division of the command and therefore allow the resolution of the system of equations.
In a particular embodiment of the invention, the preponderant offset measured by the control sensor is caused by a physical phenomenon that it is worthwhile quantifying since its value, usually measured by an ancillary system, is used in the definition of the setpoint of the system.
For example, in a backlighting command of a liquid crystal screen, the evolution of the offset is mainly due to the variations in the ambient lighting, described above as being the physical phenomenon. By determining a response without offset of the system, the invention makes it possible to obtain a lighting measurement that is independent of the ambient light. The offset can also be used to adapt the setpoint 11 instead of the use of an ancillary sensor for measuring the ambient lighting. This makes it possible to use only a single sensor to both control the lighting command to the setpoint, and measure the ambient lighting in order to generate the setpoint. In other words, in a more general manner, the setpoint received by the device can be a function of the measured offset. In this instance, the measurement of the offset, mainly due to the ambient lighting, is weighted and then added to the setpoint. For example, the lighting setpoint is increased when the ambient light increases.
The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given as an example, which description is illustrated by the appended drawing in which:
For the purposes of clarity, the same elements will bear the same references in the various figures.
A method according to the invention is adapted to a time-division multi-level command. This command is for example cyclical and inside a cycle there follows an active phase during which the system is commanded to a maximum level and an inactive phase during which the system is commanded to a minimum level, for example zero. This type of command is called command by pulse width modulation. The timing chart of
According to the invention, the method comprises the following operations:
The timing chart of
In the example illustrated by the timing chart of
Two other exemplary embodiments of a method according to the invention are illustrated by the timing charts of
In the example illustrated by the timing chart of
The measurement 33 can be expressed as follows:
Mlong=Rsystem+Offsetcycle (1)
Mlong representing the measurement 33, Rsystem representing the response of the system and Offsetcycle representing the offset over a complete cycle.
The measurement 34 depends on the duty factor. If this factor is less than 50%, the measurement 33 can be expressed as follows:
Mshort=Offset1/2 cycle (2)
Mshort representing the measurement 34, and Offset1/2 cycle representing the offset over half of the cycle. If the duty factor is greater than 50%, the measurement 33 can be expressed as follows:
Mshort=Rsystem−½Rsystem max+Offset1/2 cycle (3)
Rsystem max representing the response of the system for a duty factor of 100%.
In both cases, duty factor being less than or greater than 50%, the response of the system Rsystem can be expressed as a function of:
Mlong−2×Mshort (4)
Specifically, if the duty factor is less than 50%, this gives:
considering that the offset is constant over the whole cycle and therefore that:
Offsetcycle=2×Offset1/2 cycle (7)
Moreover, if the duty factor is greater than 50%, this gives:
and therefore:
Mlong−2×Mshort=Rsystem max−Rsystem (9)
The response of the system Rsystem max for a duty factor of 100% being a constant known elsewhere, it is easy to determine the effective response of the system Rsystem. This response is not subjected to the offset.
Based on the response of the system, determined previously, it is easy to determine the offset if necessary from one of the two measurements 33 or 34. For example, from the measurement 33 the equation (1) is used:
Offsetcycle=Mlong−Rsystem (10)
The offset thus defined can be used to define the command 11 for example in the case of an embodiment of the invention for the backlighting of a screen considering that the preponderant offset is associated with the ambient light.
The curve 41 represents in the same coordinate system the difference between the measurement 33, Mlong, and twice the measurement 34, Mshort, as a function of the duty factor PWM in the form of a curve 41 as a thick line. The curve 41 is formed of two line segments 44 and 45. The segment 44 extends from the origin 42 of the coordinate system to a point 46 linking half of the maximum response of the system, ½ Rsystem max, to a duty factor of 50%. The segment 44 is superposed on the curve 40. The segment 45 extends from the point 46 to a point 47 linking a difference between the measurement 33, M long, and twice the measurement 34, Mshort, zero to a duty factor of 100%.
In the example illustrated by the timing chart of
A variant of a method according to the invention consists in taking the two measurements during one and the same cycle. In this variant, the two measurements may or may not overlap. The overlap will occur in a mandatory fashion in combination with the variant described with the aid of
Ginestet, Thierry, Come, Sylvain, Chaillou, David
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